I would just LOVE this card, but I think that Golems should be toned down a bit...
Also, Gravity is about high hp creatures, right? So, why don't you make it 1/2 and 2/3 creatures? You may want to lower the price for it too... Something like 4
unupped, 3
upped? With golem costing 1
for a 1/2 (playable even alone) and mythril golem being a 2/3 for 2
.